What you need is a goodness-of-fit test for homogeneity. The most well-known goodness-of-fit test is the Chi-square. A somewhat better one is the G test. Both are described in Sokal and Rohlf. In tests for homogeneity, your 'null hypothesis' is given by the proportions of the various species when you lump all your samples together.
Note that these tests will test for overall differences in proportions among all species. When you have many species, the test almost always comes out significant, as the null hypothesis (no differences in ANY species) is so unlikely. So you should always combine the significance test with a measure of the MAGNITUDE of the difference, for which you can use one of the many community comparison indices (concordance, Morisita-Horn). Final point: your idea to test species proportions separately is not valid, for two reasons. One is that, with proportions, your species values are not independent -- if one species is a very high proportion of one community, then the other speceis must necessarily have relatively low proportions. The other reason is that you will be making very many separate tests, and it is likely that come out significant purely by chance. Gareth On Mon, 21 May 2007 14:16:53 -0700, Bethwell Moyo <[EMAIL PROTECTED]> wrote: >Dear all, > > I have data on number of certain species in a rotational grazed plot and > also the number of same species in a continuosly grazed one collected for 8 years. The idea is to check whether there is a change in their proportion as year progressed. I have tried repeated measures, but was discouraged. A suggestion to compare proportions was put forward, and i don`t know how to test significant differences in proportions between the two plots. Part of the data looks like this: > year Species Continuous Rotational > Prop_Cont Prop_Rot 1992 AB 440 701 0.0484 0.0842 1992 BE 110 165 0.0121 0.0198 1992 BI 308 218 0.0339 0.0262 1992 CP 764 1571 0.0840 0.1886 1992 CV 296 428 0.0325 0.0514 1992 DM 927 681 0.1019 0.0818 1992 ES 114 131 0.0125 0.0157 1992 ESP 531 659 0.0584 0.0791 1992 FB 810 739 0.0891 0.0887 1992 HC 3825 1624 0.4206 0.1950 1992 PN 42 37 0.0046 0.0044 1992 RN 135 47 0.0148 0.0056 1992 SI 177 73 0.0195 0.0088 1992 TB 247 559 0.0272 0.0671 1992 TT 348 694 0.0383 0.0833 1992 UM 21 3 0.0023 0.0004 1992 Total 9095 8330 1993 AB 1031 1270 0.0328 0.0464 1993 BE 8049 4673 0.2557 0.1706 1993 BI 184 318 0.0058 0.0116 1993 CP 680 1333 0.0216 0.0487 1993 CV 2163 1679 0.0687 0.0613 1993 DM 372 444 0.0118 0.0162 1993 ES 71 128 > 0.0023 0.0047 1993 ESP 709 474 0.0225 0.0173 1993 FB 3913 3946 > 0.1243 0.1441 1993 HC 4063 1925 0.1291 0.0703 1993 PN 4558 4891 0.1448 0.1786 1993 RN 408 367 0.0130 0.0134 1993 SI 171 520 0.0054 0.0190 1993 TB 4481 4803 0.1423 0.1754 1993 TT 160 406 0.0051 0.0148 1993 UM 467 209 0.0148 0.0076 1993 Total 31480 27386 > > > How do I show that for example there has been a signicant change in > proportion of species AB, in 1993. I want a statistical method of dertemining that, especially using proportions. > > Thanx for the help > > > ============================================================ ======= > It is only with the heart that one can see rightly. What is essential is > invisible to the eye. >Mr B Moyo > Fixed +27 40 602 2123 > Mobile +27 72 464 3759 > Fax +27 86 517 2499 > ============================================================ ======= > > > >--------------------------------- >Take the Internet to Go: Yahoo!Go puts the Internet in your pocket: mail, >news, photos & more. >=========================================================== ==============
